What is the capacity factor at the site?

Energy yield calculations are based on site specific information, windfarm capacity and must also take into account what would be considered to be a reasonable operating regime for the wind turbines based on network and market restrictions. Initial work has identified that a windfarm can be accommodated at the site. The windfarm would be operated by Manx Utilities and not subject to the curtailment conditions which prevail in a regulated market such as in Great Britain. Both of these factors have a material effect on the energy yield which could be anticipated from a windfarm at Earystane (Cair Vie Windfarm).

The UK’s own figures suggest that the average load factor of projects commissioning in 2023 is 45%. Our source of this data is from a 2023 report issued by the UK’s DESNZ load factor assumptions.

DESNZ advise that load factors, defined as expected annual generation as a percentage of theoretical maximum generation, are modelled to increase with turbine size. Larger turbines are expected to produce higher load factors for several reasons, most importantly that larger turbines can access higher winds due to their increased height, and that a windfarm with fewer, larger turbines has increased efficiency. Detailed discussion of these relationships can be found in a report for the department by DNV GL Energy. DESNZ further states that future load factors were calculated by combining a theoretical turbine power curve (power output as a function of wind speed, modelled using turbine specifications provided by manufacturers) with hourly wind speed data from existing windfarm sites.

There is a cubic relationship between wind speed and power output from turbines; this means that sites with even slightly higher wind speeds have a significantly higher output. For example, a windfarm at a site with an average wind speed of 7m/s would be expected to generate 25% more power than a windfarm with average wind speed of 6m/s. Actual measured and 30 year historic wind speeds at Earystane are well above average for an onshore windfarm in the UK.

The UK figures also include curtailment and most projects have been commissioned on sites with much lower wind speeds than those which have been demonstrably measured at Earystane. However, remote island windfarm projects (such as the Isle of Man or the Shetland Isles) have higher load factors than other inland onshore windfarms. This is because of the higher average wind speeds and higher availability of the wind resource.

The Shetlands project commissioned in 2000 and 2003 had an average capacity factor of 53% over the last five years. Technologies have improved significantly since this windfarm was installed, which would lead to higher load factors for projects commissioning today.

A windfarm at Earystane could generate enough electricity to meet over a quarter of our Island’s annual electricity demand. This has been determined by wind turbine design experts directly by taking the average wind speeds across every 10 minutes over the last five years based on data calculated by the UK Met Office using their Hadley Centre Model to transpose data from elsewhere on the Island to the Earystane site. This data has been used alongside a manufacturer’s performance curves, an example of a performance curve is shown below:

Manx Utilities is interested in Class 1 (or S) turbines at Earystane, which are specifically designed to operate in higher wind conditions. The cut off speeds for Class 1 wind turbines typically range from 26 to 28m/s: beyond this wind speed, wind turbines will not generate any power. Based on actual wind data at Earystane, this is expected to occur no more than 90 hours per year and has been factored into the energy yield calculations.

The cut in speeds for Class 1 wind turbines typically range from 2 to 4m/s: below this wind speed, there is insufficient force available to rotate the turbine blades and wind turbines will not generate any power. Based on actual wind data at Earystane this is expected to occur no more than 500 hours per year and has been factored into the energy yield calculations.

The energy yield calculations also account for localised turbulence which may impact individual turbines and reduce windfarm output. They do not at this stage account for availability or down time due to network outages which may theoretically reduce the figures but this is not anticipated to be significant.

Occasionally capacity factor and load factor (LF) are confused.  In some cases, they can have a very similar meaning but they are subtly different.

Load factor (LF) is a measure of a plants performance and is calculated as actual output over a year divided by the theoretical maximum output over a year.  A thermal generator (such as coal, gas or nuclear), which operates for an entire year with no breakdowns or maintenance outages could achieve a load factor of 100%.

Capacity factor instead is used to measure performance of renewable generators notably solar and wind. The calculation is essentially the same as for load factors. It is actual output over a year divided by the theoretical maximum output over a year.  Capacity factor will never approach 100% since to do so would require the sun to remain directly overhead and for wind to blow at gale strength continuously for the entire year.

It is not accurate to calculate energy yield based on data derived from sites elsewhere as each site is unique, hence our calculations rely on real wind and performance data for this site only. It is not assumed that any curtailment will be required for Cair Vie given the maximum output is lower than the current minimum demand. The capacity factor based on this yield would be expected to be 64% without curtailment and this capacity factor is calculated directly from the expected energy yield.

The wind speeds which have been measured so far correlate well with the Met Office dataset used to calculate energy yields giving reasonable confidence in the results at this stage.

It should be remembered that this project is at the development phase which includes data collection and analysis to validate or modify assumptions made in earlier phases of the project. Once a full year’s on-site data has been collected and a turbine supplier has been selected, more detailed analysis of the yields can be undertaken, although the results are not expected to differ dramatically from the predictions to date given our current state of knowledge.